[libc++] NFC: Move the status docs to their own subdirectory

This cleans up the libcxx/doc directory quite a bit and will avoid the
proliferation of status files in that directory as new standards are voted.

NOKEYCHECK=True
GitOrigin-RevId: cf005c4c5061ed34b2bd21ae104923c28b8a1b04
diff --git a/docs/index.rst b/docs/index.rst
index 7e5cb08..9cb7fc7 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -39,12 +39,12 @@
    BuildingLibcxx
    TestingLibcxx
    Contributing
-   Cxx1yStatus
-   Cxx1zStatus
-   Cxx2aStatus
-   Cxx2bStatus
-   RangesStatus
-   FormatStatus
+   Status/Cxx14
+   Status/Cxx17
+   Status/Cxx20
+   Status/Cxx2b
+   Status/Ranges
+   Status/Format
 
 
 .. toctree::
@@ -139,9 +139,9 @@
 ---------------------
 
 * C++11 - Complete
-* :ref:`C++14 - Complete <cxx1y-status>`
-* :ref:`C++17 - In Progress <cxx1z-status>`
-* :ref:`C++20 - In Progress <cxx2a-status>`
+* :ref:`C++14 - Complete <cxx14-status>`
+* :ref:`C++17 - In Progress <cxx17-status>`
+* :ref:`C++20 - In Progress <cxx20-status>`
 * :ref:`C++2b - In Progress <cxx2b-status>`
 * `Post C++14 Technical Specifications - In Progress <http://libcxx.llvm.org/ts1z_status.html>`__
 * :ref:`C++ Feature Test Macro Status <feature-status>`